Among the known illuminated ring devices employed in observing the topography of the corneal surface must be included the original 1880 disc of Placido and the keratometer of Kilmer disclosed in U.S. Pat. No. 3,797,921 both of which used rather large hemispherical targets as well as the device of Knoll U.S. Pat. No. 3,248,162 which used a comparatively large diameter cylindrical target. An alternative to these large target devices is disclosed in U.S. Pat. No. 4,772,115 to Martin Gersten, Richard J. Mammone, and Joseph Zelvin. That device included a conical structure of translucent plastic having an opaquely coated hollow axial bore or passage whose diameter was only slightly larger than that of the object, typically the cornea of a patient's eye which was to be positioned adjacent to the bore at the cone's apical end. The opaque coating of the cylindrical bore was incised with a plurality of ring-shaped cuts to allow light which entered from a light box at the cone's base to illuminate the incised rings. An observer, or properly focused camera, looking into the bore's opposite end could acquire an image of the ring pattern appearing on, i.e., reflected from, the object. Variations in the radius of the rings in the pattern from circularity represent distortions of the curved surface.
While the aforementioned conical device has achieved a certain measure of success, the array of incandescent lamps in the lightbox which was required to provide sufficient illumination of the rings generated a considerable amount of heat. Also, the controlling the thickness of the opaque coating and the incising of the coating in the bore to define the light transmitting rings have proven to be difficult operations to control precisely; sometimes the coating would chip or be scratched off during processing or in the course of use. As a result, some rings were not precisely formed, and their images as reflected from the corneal surface were indistinct.